The present invention relates to a process for continuously producing a propylene-based block copolymer having an excellent impact resistance.
As methods of improving an impact resistance of crystalline polypropylene, there are known (1) a method of copolymerizing propylene with .alpha.-olefin which co-exists therewith in a small amount upon the polymerization of propylene, (2) a method of mechanically blending a rubber component with polypropylene, (3) a method of polymerizing propylene as a primary component and then copolymerizing propylene with .alpha.-olefin other than propylene, that is, carrying out a so-called block copolymerization, or the like.
However, in the case of the above method (1), there arises a problem that although the impact resistance of the resulting copolymer is improved, the rigidity thereof is considerably lowered so that the balance between the rigidity and the impact resistance is deteriorated. Further, in the case of the above method (2), although the balance between the rigidity and the impact resistance is improved, this method is disadvantageous in production costs because the rubber component blended are usually expensive.
Under these circumstances, the above block copolymerization method (3) is used as the most effective method for improving an impact resistance of crystalline polypropylene. In this block copolymerization method, a rubber-like component which is constituted by blocks composed of a copolymer of propylene and ethylene or the like, is produced in the presence of a resinous component constituted by blocks composed primarily of propylene. Both the components are blended with each other by being continuously subjected to the polymerization. The block polymerization method, which is referred to as "chemical blend", is favorably used.
Meanwhile, in order to reduce the production costs, the block copolymerization is carried out more advantageously by a continuous process in which the respective blocks are produced in separate polymerization reactors (if required, each block can be produced in two or more different polymerization reactors), than a batch process.
However, in the case of the process using an excessively large number of polymerization reactors, there arise disadvantages such as increase in installation costs or running costs. Therefore, it is desirable to conduct each block in a single polymerization reactor. In the case of the production of the block copolymer composed of two different blocks A and B, it is most desirable to produce the block copolymer in a continuous process using two reactors. In such a case, as conventionally known, a first reactor is used to prepare the resinous component composed primarily of propylene and a second reactor is used to prepare the rubber-like component.
Incidentally, there is a tendency that propylene-based block copolymer obtained by a simple continuous process using two reactors is deteriorated in impact resistance as compared with the block copolymer obtained by a batch process. It is suggested that the difference in impact resistance therebetween is caused due to the following mechanism.
That is, in each reactor used in the continuous process, there is a difference in residence time between respective reaction materials, i.e., a distribution of residence time thereof exist. For this reason, in the simple continuous process using two reactors, considerably large amount of polymer particles and catalyst particles are transferred from the first reactor to the second reactor before these materials remain in the first reactor for a necessary residence time (such a phenomenon is called "short pass"). The particulate mixture of polymer and catalyst particles short-passed through the first reactor cannot contain a sufficient amount of resinous component due to a short residence time in the first reactor and in addition exhibits a higher polymerization activity than those remaining in the first reactor for the defined residence time. The short-passed polymer particles turn into polymer particles having a higher rubber content than required in the second reaction. A rubber-like component contained in such a high rubber content polymer particles causes deteriorated dispersion when subjected to melt-mixing, resulting in forming so-called fish eyes in shaped products. The fish eyes thus formed are apt to cause centers of breakage of the molded product because excessive stress is concentrated at boundary surface portions thereof. Accordingly, it is considered that the resultant block copolymer unsuitably suffers from deterioration in impact resistance, especially low-temperature impact resistance.
In view of the afore-mentioned mechanism, in order to prevent the deterioration in impact resistance, it is effective to suppress the occurrence of such "short pass" of the polymer and catalyst particles. In order to attain this object, for example, Japanese Patent Publication (Kokoku) No. 49-12589 (1974) has proposed a process of using, in the afore-mentioned first polymerization stage, a plurality of polymerization reactors which are connected in series with each other. However, such a process is a multiple reactors process and, therefore, become disadvantageous in costs as described above.
Further, since an alternative method of suppressing the "short pass" of the particles in the first reactor, there is known a process of using a classification system. For example, Japanese Patent Applications Laid-open (Kokai) Nos. 51-135987 (1976) and 55-116716 (1980) disclose a process of subjecting the polypropylene particles in the first reactor to concentration and classification processes, thereby removing therefrom the catalyst particles short-passed through the first reactor or polypropylene particles having a small particle size due to insufficient growth. Furthermore, Japanese Patent Application Laid-open (Kokai) No. 55-106533 (1980) discloses a process in which a polymer slurry is discharged while stirring from the first reactor, then subjected to counter-flow washing, thereby producing a slurry containing substantially few amount of polypropylene particles having a small particle size, and then supply such a slurry to the second copolymerization reactor. However, in such a simple classification system, the impact resistance of the final product cannot be sufficiently improved.
In addition, Japanese Patent Application Laid-open (Kokai) No. 7-286004 (1995) discloses a method of continuously producing an olefin polymer by a classification system using a cyclone. However, such a method has the following problem. First, in the method, particles having a small particle size, especially fine particles having a diameter of less than 75 .mu.m, cannot be separated sufficiently, resulting in insufficient effects of suppressing the "short pass" of the polymer and catalyst particles in the first reactor. Second, in the case of the classification system using a cyclone, when the recycling percentage of the particles is increased to enhance its classification efficiency, there arise a problem that the productivity is considerably deteriorated.
On the other hand, apart from the afore-mentioned viewpoint of suppressing the "short pass", there is also known a method for improving the impact resistance of propylene-based block copolymer by supplying some kinds of assistants to the reaction system in a rubber polymerization stage. For example, Japanese Patent Application Laid-open (Kokai) No. 55-115417 (1980) discloses a method of supplying tertiary amine, ketone, ether, ester, acid amide or phosphoric acid amide; Japanese Patent Application Laid-open (Kokai) No. 57-147508 (1982) discloses a method of supplying a halogen-containing aluminum compound; Japanese Patent Application Laid-open (Kokai) No. 61-69821 (1980) discloses a method of supplying alcohol; Japanese Patent Application Laid-open (Kokai) No. 62-116618 (1987) discloses a method of supplying glycol ether; and Japanese Patent Application Laid-open (Kokai) No. 1-152116 (1989) discloses a method of supplying hydrogen sulfide.
However, when a magnesium compound supported highly active catalyst is used, the addition of such assistants cannot improve the impact resistance to a satisfactory extent. Specifically, when it is intended to improve the impact resistance, it is necessary to considerably reduce a polymerization activity of the rubber-like component. For this reason, there is caused a large gap between the polymerization activity to the rubber-like component in the second reactor and the activity of the propylene polymer in the first reactor, resulting in deteriorated productivity.
Thus, the afore-mentioned conventional techniques still exhibit various problems with respect to costs and properties of the products.
As a result of various studies by the present inventors, it has been found that by using a magnesium compound supported highly active catalyst which has a sharp particle size distribution, the short pass of the catalyst in the first polymerization stage is effectively suppressed, a propylene-based block copolymer having a good impact resistance can be produced with low costs. The present invention has been attained on the basis of this finding.